1. Field of the Invention
The present invention is concerned generally with data recording systems, and more particularly with systems for recording graphic information on a moving record medium.
2. Description of the Prior Art
Many types of graphic data recording systems are in commercial use today, including ones in which information is recorded photographically, electrographically, xerographically, magnetically, and by other equally well-known techniques. In one of the oldest and perhaps most widely used methods, a sheet of paper or other suitable record medium is moved past an electrically- or electromagnetically-driven writing element, such as an inked pen, a heated stylus or the electron beam of a cathode-ray tube (CRT). Electrical signals representative of data to be recorded cause the writing element to be deflected across the moving record medium to produce a line or series of marks forming a graphic image. As is well known, however, it is not possible to obtain highly accurate and uniform images with such a system unless the record medium travels at a constant speed relative to the rate information is written on it.
One way to minimize variations in the travel rate of a moving record medium is to use a very high quality drive mechanism that includes a constant speed motor, precision gearing, and an accurately machined transport assembly. The cost of such mechanisms generally restricts their use to low volume, special purpose applications.
An alternative approach is to compensate in some manner for velocity fluctuations caused by motor speed changes, friction, gear eccentricity and other conditions that affect the medium travel rate. Several record medium velocity compensation methods have been described in earlier issued patents. For example, U.S. Pat. No. 3,701,995 to Geerling describes a system used in a time domain reflectometer (TDR) cable tester to ensure that graphic images produced by the instrument's chart recorder correlate with those displayed on its CRT. Included in the system are means for generating a series of fixed-duration pulses at a rate proportional to the movement of the chart recorder's record medium. The pulses are converted to a stairstep analog signal that serves as a reference for the generation of a slow-rise voltage ramp having an identical slope. The slope of the ramp signal, which is used both for horizontal deflection of the CRT's electron beam and for timing the sampling of input signals received by the cable tester, thus is controlled by the travel rate of the record medium, eliminating the effect of variations in its velocity.
U.S. Pat. No. 4,172,259 to Lowe discloses a graphic recording system in which a light sensitive record sheet is drawn across the outer surface of a fiber optic strip set in the faceplate of a CRT. Images are written on the sheet by scanning the CRT's electron beam back and forth along the phosphor-backed inner surface of the strip and intensity modulating the beam in accordance with the data to be recorded. To compensate for irregular or uneven movement of the light-sensitive sheet, the disclosed system includes means for sensing the sheet's instantaneous velocity and for deflecting the electron beam in the direction of sheet movement as it is being scanned in a substantially perpendicular direction along the fiber optic strip. The beam is deflected to the extent necessary to eliminate the effects of short term velocity changes on the recorded images.
U.S. Pat. No. 3,395,401 to Silverman is concerned with digital information recording systems of a type wherein the information to be recorded is received at irregular intervals, and the record medium is advanced intermittently to record the information at uniformly-spaced locations along the medium's direction of travel. Limits on recording speed imposed by the inertia of the record medium and its drive system are overcome, according to the patent's teachings, by shifting the recording means (e.g., a light beam or magnetic tape recording head) along the medium's direction of travel concurrently with the intermittent advancement of the medium.
Finally, U.S. Pat. No. 3,893,616 to Trousdale describes a digital controller that extends the performance capability of digitally controlled electromechanical plotters by regulating the rate at which plot commands are transferred from a data source to the plotter. The controller feeds the plot commands to the plotter at established optimum velocity and acceleration rates for the device.